1. Field of the Invention
The present invention relates, in general, to detector arrays and, more particularly, to a two component configuration of an X-ray photodiode detector sensor array having a flip chip bonded application specific integrated circuit.
2. Relevant Background
Modern X-ray computer tomography (CT) scanners commonly employ several hundred X-ray detectors to convert X-ray energy into visible light and ultimately into electrical signals. A detector is usually composed of a scintillator to convert X-ray energy into light and a photodiode to convert that light into an electrical current. The formats of photodiodes used in CT applications can range from a single element, 1-D array to a multi-element, 2-D array.
Each active photodiode array comprises a series of scintillation crystals arranged on a substrate for converting X-ray radiation into light. Under each scintillator crystal is a back-illuminated photodiode that converts the light emitted from the scintillation crystals into an electrical charge. The electrical charge from the photodiodes is then conveyed via an electrical path to a signal processing circuit. Typically, the converted electrical charge leaves each photodiode via electrical connections through a plurality of paths in a substrate to the processing circuitry using a wire bonding technique as is commonly known in the art. The substrate serves both as a supporting mechanical foundation for the circuitry and the photodiode assembly, and as a means to house a shield to protect the processing circuitry from stray radiation.
FIG. 1 provides a plan and side view of a highly abstract rendition of a typical photodiode array 100 as is known in the art. A scintillator crystal 110 is typically a six-sided cube that receives X-ray radiation 105 wherein the one transparent face is bonded to a photodiode 140. The juncture between the scintillator crystal 110 and the photodiode 140 is normally a p+ on n− mating. The remaining sides of the crystal 110 are covered with an optically reflective material that facilitates channeling the light generated by the crystal to the transparent face and ultimately to the photodiode 140 below. The photodiode is thereafter connected to processing circuitry 160 via electrical paths amidst a substrate 150. Interposed between the substrate 150 and the processing circuitry 160 is a radiation shield 145. The shield 145 protects the processing circuitry from stray radiation that may result in false readings, imaging, and ghosting.
As illustrated schematically in FIG. 1, a substrate 150 provides the basic structural support of the prior art detector array. Photodiodes 140 are arranged on the substrate 150 in a 2-D array. The substrate 150 also includes a signal transmission means 170 for transmitting electrical signals generated by each photodiode 140 to the signal processing circuitry 160 for image reconstruction. The processing circuitry associated with each electrical signal from each active photodiode element is typically a Complementary Metal-Oxide Semiconductor (“CMOS”) chip. CMOS is a major class of integrated circuits. CMOS chips include microprocessor, microcontroller, static RAM, and other digital logic circuits. A wire bond typically connects a top surface bond pad on one end of the photodiode to an external connection on the CMOS chip.
The signal transmission arrangement typically includes electrically conductive circuit paths 170 (wires) printed into the substrate. Electrically conductive leads from each photodiode 140 to one or more of the paths 170 complete an electrical connection between each photodiode 140 and the processing circuitry 160. As the number of photodiodes 140 in each array grows, the complexity of the substrate supporting the numerous electrical wires required for the array also expands. As will be appreciated by one skilled in the art, substrates can comprise of multiple layers to accommodate in excess of 500 individual circuit paths.
As will also be appreciated by those skilled in the art, manufacture of the prior art X-ray detectors such as that shown in array FIG. 1 tends to be complex and labor intensive due to the precautions necessary to insure its reliable construction. For example, each connection 170 between each photodiode 140 and the processing circuitry 160 must be bonded, and carefully looped, such that the wire 180 when bonded to a circuit path on the substrate 150 does not interfere with other wires or other conductive areas on the substrate 150. The wire bond density becomes acute for 2-D arrays. A conductive trace from each inner photodiode element in a 2-D array must be connected to the “outside world”. One trace is required per element and each trace usually terminates in a bond pad on substrate 150. Wire bonds from each bond pad are then made to external connections.
As the number of these external connections increase so does the cost associated with the substrates associated with the photodiode array. Until recently, the size and number of the photodiodes in the detector array was normally the most significant factor in the design of such components. Advancements in bonding and the ability to position large numbers of detectors in a single array has shifted the allocation of costs of these components to the design and manufacturing of the substrates and their ability to carry hundreds of independent electrical signals. Substrates capable of conveying the signals for some 500 diodes are often on the order of 10-12 layers thick. To create such a substrate that is not only reliable but also possesses sufficient electrical leads and has thermal expansion characteristics consistent with the photodiode detector array to which it is bonding is difficult and expensive.
Therefore, it is an object of the present invention to provide a 2-D photo-detector array with a simplified structure leading to lower manufacturing costs. It is a further object of the invention to provide an improved X-ray detector having improved component verification, greater durability, and reliability, which is substantially less costly to manufacture than prior art X-ray detectors.